The invention relates to a light modulator having a photochromic layer, which can be activated optically by control light, for modulating signal light, and at least one optically transparent substrate for the photochromic layer.
Light modulators of the aforementioned type are also denoted as optically addressable, spatial light modulators. Although the photochromic layer is xe2x80x9caddressedxe2x80x9d optically only two-dimensionally and not three-dimensionally, it is usual to talk of a spatial light modulator instead of a planar one. Such light modulators are denoted below as OASLMs.
The photochromic layer serves the purpose of transmitting or conveying information from the control light to the signal light. In the event of irradiation with control light of a predetermined first optical wavelength, the photochromic layer reacts at the site of irradiation with a change in specific optical propertiesxe2x80x94in particular with a change in the optical absorptivityxe2x80x94for signal light of a predetermined second optical wavelength. For example, the control light can be used to project an intensity contrast image onto the photochromic layer, which then reacts with a setting, corresponding to the contrast image, of its absorptivity as regards the signal light beyond the area irradiated by the control light. If the photochromic layer thus activated optically by control light is irradiated with signal light, the signal light emerging from the photochromic layer has a modulation corresponding to the absorption contrast pattern. Information from the control light can therefore be transmitted onto the signal light in a planar fashion in a way which varies with time. The signal light striking the photochromic layer can be an extended light bundle which simultaneously covers the entire light entry area of the photochromic layer. A corresponding statement holds for the control light. However, it is also possible to xe2x80x9cwritexe2x80x9d the relevant information into the photochromic layer with the aid of a deflectable control light beam. Likewise, it is also possible for the purpose of xe2x80x9creadingxe2x80x9d or xe2x80x9cerasingxe2x80x9d the information to make use of a signal light beam or xe2x80x9cerasing light beamxe2x80x9d which, for example, scans the photochromic layer by row or by column.
A multiplicity of photochromic materials come into consideration for such applications. An overview of the essential photochromic material classes, their best known representatives and their properties is to be found in H. Dxc3xcrr, H. Bouas-Laurent, xe2x80x9cPhotochromismxe2x80x94Molecules and Systemsxe2x80x9d, Studies in Organic Chemistry, Elsevier, Vol. 40, 1990. In addition to other photochromic materials such as, for example, synthetic inorganic and organic photochromics, bacteriorhodopsin in the form of a-purple membrane, denoted below as BR, is a particularly interesting material for forming the photochromic layer. Purple membrane is the form used for the naturally occurring two-dimensional crystalline form of bacteriorhodopsin. The design of the so-called purple membrane from lipids and bacteriorhodopsin is described in numerous examples in the literature. D. Oesterhelt et al., Quart. Rev. Biophys., 24 (1991) 425-478 may be cited by way of example as a reference.
It is, inter alia, the following five reasons which render bacteriorhodopsin particularly suitable for the application outlined.
(i) BR is distinguished by a very efficient photochemical reaction with several photoactive states which render it possible to implement xe2x80x9cwritingxe2x80x9d and xe2x80x9cerasingxe2x80x9d photochemically.
(ii) BR has a particularly high reversibility, and this predisposes it to dynamic use.
(iii) The specific absorptions of the long lived states of BR, and also the difference in refractive index between these states are very high, and so good modulation of the signal light is achieved.
(iv) Bacteriorhodopsin has a strongly anisotropic chromophore and is therefore suitable for polarization-selective modulation.
(v) Apart from the wild type of BR, there is currently available a whole series of variants of BR produced using gene technology and having altered amino acid sequences and/or variants, which contain as chromophores molecules differing chemically from retinylidene radical and have other spectral and/or other photokinetic properties than the wild type, for example different absorption properties and/or substantially longer lived photointermediates.
The material group specified in (v) is denoted below as BR variants. The term bacteriorhodopsin or BR is used in such a way that either the wild type of bacteriorhodopsin or one of the BR variants is understood thereby. Furthermore, the term bacteriorhodopsin or BR is used both for monomeric BR and for BR in the form of purple membranes. BR variants may be obtained with various methods. An overview of known methods for producing mutated bacteriorhodopsins and BR analogs, which are typified by the presence of chromophore groups differing from the retinylidene radical occurring in the wild type, is given in N. Vsevolodov, xe2x80x9cBiomolecular electronicsxe2x80x94an introduction via photosensitive proteinsxe2x80x9d (1988), Birkhaxc3xcser, Boston, Chapter 3. Typical BR variants of technical interest which are obtained by modifying the amino acid composition of wild type BR are those with a lengthened lifetime of the so-called M state, for example those in which the aspartic acid at position 96 has been replaced or removed or displaced in its position by removal of other amino acids, or those with a high probability of the formation of 9-cis-retinal, for example those in which the aspartic acid at position 85 has been replaced or removed or displaced in its position by removal of other amino acids. Typical technically interesting BR variants which are produced by replacing the retinylidene radical occurring in wild type BR by analog molecules are, for example, 4-ketoretinal and dihydroretinal (Sheves et al., Biochem., 24, 1985, 1260-1265). It may be pointed out expressly that a combination of modifications of the amino acid composition and replacement of the chromophore group is also understood by the term BR variants.
Said possibilities and properties of BR are known to the person skilled in the art and have also influenced applications of BR in various optical information processing techniques.
The optically active component is formed by the BR layer in OASLMs. The optical modulation is based on the fact that bacteriorhodopsin can be converted from the initial state B (maximum absorption at approximately 570 nm) by irradiation of light of wavelength xcexB into at least one other spectrally different state. The longest lived state of the photocycle of wild type BR is usually denoted as M state (maximum absorption at approximately 410 nm). Light of wavelength xcexM can be used to convert said state photochemically into the initial state B. Consequently, light in the wavelength region of xcexB can be varied and/or controlled, or else vice versa by simultaneous illumination of the BR layer with light in the wavelength region of xcexM, using the BR layer as a mediator.
The degree of modulation depends in this case on the magnitude of the photochromic optical absorption changes which is caused by the irradiation of light in the BR layer, on the quantum yield of the phototransformations BM and the intensities and wavelengths of the two optical irradiations. Because of the polarization-sensitive photoreaction of the BR, the relative position of the polarization states of the two wavelengths or wavelength regions likewise plays a role in the level of the modulation. Furthermore, the local refraction index, which can likewise be used for modulation purposes, is modulated in a manner proportional to the absorption modulation.
OASLMs have been known for a long time as active optical components in beam paths for the purpose of optical processing of images and information, and are used to control and/or to modulate the amplitude, the phase and, if appropriate, also the polarization of a spatially extended lightwave field as a function of the intensity of a control light source.
An overview of the state of knowledge concerning BR, and the possibility of applying BR in optical information processing can be gathered, inter alia, from the articles by D. Oesterhelt et al., Quart. Rev. Biophys., 24 (1991) 425-478, D. Zeisel and N. Hampp, J. Phys. Chem., 96 (1992) 7788-7792, N. Hampp et al., Proc. SPIExe2x80x94Int. Soc. Opt. Eng., 1732 (1993) 260-270 and N. Vsevolodov, xe2x80x9cBiomolecular electronicsxe2x80x94an introduction via photosensitive proteinsxe2x80x9d (1998), Birkhaxc3xcser, Boston.
The use of a spatial light modulator in a beam path for the purpose of holographic writing and reading of optical data which are stored in a BR layer is described in R. R. Birge et al., Ann. Int. Conf. IEEE Eng. Med. Biol. Soc. 12 No. 4, (1990), 1788-1789.
A spatial light modulator which is based on a BR layer and has been used as a spatial frequency filter for optical image correction, in particular for optical edge reinforcement is likewise described in the article by R. Thoma et al., Opt. Lett. 16 (1991) 651-653.
A specific spatial light- modulator based on a Perot-Fabry resonator, which contains a BR layer as active element, is described in U.S. Pat. No. 5,618,654.
The known light modulator has two plane-parallel, semitransparent mirrors situated opposite and parallel to one another. With the given mirror spacing L and refractive index n of the medium between the mirrors, the Fabry-Perot interferometer corresponding to the resonance condition L=N xcexir/2 n is virtually completely transparent to light of wavelength xcexir although, viewed individually, the mirrors must have a high reflectivity to light of the resonance wavelength xcexir. In the case of the subject matter of U.S. Pat. No. 5,618,654, the refractive index of the photochromic layer between the resonator mirrors is varied by irradiation with control light of wavelength xcexv in order optionally to fulfill the resonance condition for signal light of wavelength xcexir. In this way, the transmittivity of the interoferometer light modulator is varied overall for the signal light xcexir, and thus the signal light is modulated. In order to enable the control light required for changing the refractive index to reach the photochromic layer, in order to function the known modulator requires the resonator mirrors to be transparent to the control light with as high as possible a transmittivity, and therefore to have as small a reflectivity as possible, whereas the reflectivity of the respective resonator mirrors to signal light must be as high as possible in accordance with the functional principle of the Fabry-Perot interferometer.
Further examples of light modulators which function using the principle of the Fabry-Perot interferometer are described in DE-A 19 35 881 and U.S. Pat. No. 4,834,511. In order to function, all these light modulators using the principle of the Fabry-Perot interferometer require precise observation of the geometrical relationships, in particular the spacing between the resonator mirrors, set to fulfill the resonance condition. Maintaining the mirrors in a mutually parallel position, and avoiding fluctuations in the spacing between the mirrors over the entire modulator area also causes problems. The previously mentioned geometrical conditions which must necessarily be observed in the known light modulators usually require a vibrationless and thermostatic design in the case of light modulators using the interferrometer principle.
Further details on light modulators employing a BR layer follow from the papers by R. B. Gross et al., Proc. SPIE-Int. Soc. Opt. Eng. 1662 (1992) 186-196 and Q. W. Song et al., Opt. Lett. 18 (1994) 1373-1375, and also H. Takei and N. Shimizu, Opt. Lett. 19 (1994), 248-250.
The invention is based on the object of developing an integrated optical component which is based on an optically addressable, spatial light modulator with improved application properties and which can be used in a versatile fashion as an active switching and/or control element in beam paths for the purpose of optical imaging, in optical display systems, in optical systems for information storage and processing, and also in holographic measuring and processing systems.
Starting from a light modulator of the type mentioned at the beginning, this object is achieved-according to the invention by virtue of the fact that the light modulator has at least one filter layer, which reflects the control light in a wavelength-selective fashion, for the purpose of retroreflecting control light which has penetrated the photochromic layer the reflecting filter layer having a reflectivity of at least 80% as regards the control light.
The control light can reach the photochromic layer from a control light entry side of the light modulator, and penetrate into the photochromic layer. The reflecting filter layer is located on the side of the photochromic layer averted from the control light entry side, and ensures that the control light is retroreflected again to the photochromic layer. As a result, the control light is utilized substantially more effectively for the photochemical conversion (photoconversion) of the photochromic material, since the control light passes twice through the photochromic layer, and the control light path in the photochromic layer is thereby doubled. In this way, the intensity-dependent degree of modulation of the photochromic layer is substantially improved. This leads to economic advantages, since it is possible to use control light sources which are of lower power and thus more cost-effective. This holds, in particular, for lasers as control light sources. Alternatively, there is a reduction in the need for BR quantity per area of the OASLM in order to achieve a prescribed degree of modulation for a given control light source. This yields economic advantages, since, in particular, genetically altered bacteriorhodopsins are expensive.
The reflecting filter layer does not, however, have only the function of efficiently utilizing the control light for optical activation of the photochromic layer, but also the function of largely separating the control light from the modulated signal light, doing so by passing the signal light modulated at the photochromic layer toward a light exit side of the light modulator, and reflecting the control light in the opposite direction as determined by the reflectivity. The modulated signal light can therefore be utilized by the control light without appreciable interference. This point of view is of particular significance if a comparatively intensive laser beam is used as control light beam to xe2x80x9cdescribexe2x80x9d the photochromic layer and visual observation of the photochromic layer is intended to be performed from the signal light exit side of the light modulator, or when a light-sensitive medium, for example a photosensitive layer, is situated in the signal light beam path downstream of the light modulator.
The light modulator according to the invention has considerable advantages by contrast with the known light modulators already addressed above, which function using the principle of the Fabry-Perot interferometer and have mirrors with as small as possible a reflectivity for the control light. These advantages include a simple design which is comparatively uncritical with regard to the dimensions of layers and spacings between the filter layers. Thermostating measures are not required with the light modulator according to the invention, since linear expansion effects do not appreciably affect the functioning of the modulator, and so the functioning of the light modulator according to the invention is not impaired by normal temperature fluctuations. Since the thickness of the photochromic layer can to a large extent be selected freely in the case of the light modulator according to the invention, the production engineering requirements placed on the observation of tolerances etc., are also slight. The greater degrees of freedom with regard to any possible fluctuations in the thickness of the layers of the light modulator according to the invention facilitate the implementation of relatively large light modulator areas.
Again, it is possible to operate with polychromic signal light in the case of the subject matter of the present invention.
The filter layer reflecting the control light is preferably arranged between the photochromic layer and the substrate, being in direct contact with the photochromic layer. The fact that the photochromic layer and reflecting filter layer are directly coupled prevents a substantial beam offset which would reduce the useful resolution of the OASLM. Thus, direct coupling of photochromic layer and reflecting layer economizes on optical components, and this leads to economic advantages and minimizes the overall size of the system.
Furthermore, because the interfering control light is removed immediately downstream of the photochromic layer, the control light component in the downstream signal light beam path can be substantially reduced, and so the signal-noise ratio can be improved. An additional improvement in the signal-noise ratio results from the fact that the number of the internal interfaces, and thus the reflection losses are reduced.
The reflecting filter layer preferably has a reflectivity of at least 99% for the wavelength of maximum reflectivity, and so it is possible to separate the control light from the modulated signal light virtually completely.
The photochromic layer -preferably contains bacteriorhodopsin as active component.
It is particularly preferred for the photochromic layer to contain a variant of the wild form of the bacteriorhodopsin, which has a higher light sensitivity and/or a longer service life of the longest lived intermediates than the wild form, and specifically in particular a variant in the case of which the amino acid position 85 is modified, or a variant in the case of which the amino acid position 96 is modified, or a variant in the case of which dihydroretinal or 4-ketoretinal serves as chromophore group, or a variant in the case of which both dihydroretinal or 4-ketoretinal serve as chromophore group and the amino acid position 85 and/or 96 are modified.
The OASLM according to the invention can, in particular, have on at least one side an antireflection layer effective over a wide band of visible light.
Furthermore, it can be expedient to apply a protective layer transparent to visible light at least to the side of the photochromic layer averted from the substrate.
In accordance with a development of the invention, it is possible to provide a second wavelength-selectively reflecting layer with a wavelength-selective reflectivity differing from the first reflecting filter layer. Essentially the same result can also be achieved with a coating when the latter has two or, possibly, even more wavelength regions in which there is a pronounced selective reflection.
In accordance with a particularly preferred development of the invention, on the side of the photochromic layer averted from the reflecting filter layer a filter layer which reflects the signal light in a wavelength-selective fashion the light modulator has and retroreflects signal light which has penetrated the photochromic layer. Such a light modulator having a respective reflection filter on mutually opposite sides of the photochromic layer constitutes an optical component which is suitable for interesting applications such as, for example, for the incoherent/coherent conversion still to be described below, or for the frequency conversion likewise still to be explained.
For various applications, interest may attach to a light modulator of the type mentioned in the beginning which has only one filter layer reflecting the signal light in a wavelength-selective fashion in order to retroreflect signal light which has penetrated the photochromic layer. Such a light modulator therefore outputs the modulated signal light to that side at which the unmodulated signal light has entered the light modulator.
The invention also relates to an optical display device having a light modulator as claimed in one of claims 1-6 as display element. The optical display device comprises a control light source for activating the photochromic layer of the light modulator with the aid of control light in accordance with the information respectively to be displayed, and a signal light source for providing the signal light which is to be modulated by the light modulator in order to visualize the information to be displayed, the light modulator being arranged in the control light beam path and in the signal light beam path in such a way that the control light and the signal light enter the photochromic layer on the side of the photochromic layer averted from the reflecting filter layer, the modulated signal light being capable of emerging on the side of the light modulator opposite the light entry side. The visual observation of the displayed information or the modulated signal light is performed from the side of the light modulator averted from the light entry side. The information displayed can be a contrast image of an object mask arranged in the control light beam path which is projected onto the photochromic layer of the light modulator.
In accordance with a preferred variant of the optical display device, the control light source is a laser, a deflecting device, in particular a biaxial scanner, being provided for the controlled deflection of the control light beam, for the purpose of planar addressing, and an intensity modulator being provided which controls the intensity of the control light beam as a function of its impingement position on the photochromic layer in accordance- with the information to be displayed.
The laser can preferably be switched between two wavelengths xcexS and xcexL which are selected such that the photochromic layer can be written with light of wavelength xcexS and can be erased with light of wavelength xcexL. In this context, writing the photochromic layer means that the photochromic layer is activated in order to change its optical properties for the signal light. Erasing the photochromic layer means in this context that the photochromic layer is returned to its original state again.
The invention also relates to an optical arrangement having a control light source, a signal light source and a light modulator as claimed in claim 7 for the purpose of transmitting information contained in a control light beam onto a signal light beam, in which the control light beam carrying the respective information enters the photochromic layer from the side of the photochromic layer of the light modulator averted from the filter layer reflecting the control light in a wavelength-selective fashion, and the signal light beam enters the photochromic layer from the side of the photochromic layer averted from the filter layer reflecting the signal light in a wavelength-selective fashion.
Such an optical arrangement can be used as an incoherent/coherent converter when the control light is incoherent and the signal light is coherent. The information contained in the incoherent control light beam can be transmitted in such an arrangement onto the coherent signal light beam. The information to be transmitted can be, for example, the image of an object mask located in the control light beam path, which image is projected onto the photochromic layer of the light modulator.
A further possibility of use for the above-named optical arrangement relates to the transmission of the object information contained in a control light beam with the wavelength xcex1 onto a signal light beam with the wavelength xcex2. In this case, the control light source is a laser with the wavelength xcex1, and the signal light source a laser with the wavelength xcex2.
Preferred applications of the invention are:
(i) high-resolution optical display systems which can be viewed with a naked eye from the light exit side of an OASLM for the modulated signal light without running the risk of being dazzled and/or injured by the intense laser light which is used as control light,
(ii) projection displays for high-resolution data projection,
(iii) incoherent/coherent converters for various optical systems, and
(iv) variable masks for the exposure of light-sensitive layers in photolithography.